This removes the special case which assumed unity gain
when reproducing mono files on a stereo system.
ITU-R BS.1770 however specifies a channel weight of 0dB for
left, right and centre, regardless of the total channel count.
Tech 3344 6.16 mentions a 3dB attenuation to maintain the
loudness level of a mono audio signal in multi-channel signals,
and Tech 3343-2016 further specifies that "Ideally, a downmix
operation should be loudness-agnostic".
